(a) Field of Invention
This invention relates to a novel design of an emergency lighting unit for providing emergency lighting from a lighting fixture employing an electric discharge lamp during periods when the normal electric power supply to the lighting fixture has failed. More specifically, the invention relates to such a novel design wherein the emergency lighting unit includes a transformer having two closely coupled windings constituting the primary of the transformer.
(b) Description of Prior Art
Electric discharge lamp lighting fixtures consist essentially of a housing, one or more lampholders to suit the number and type of lamps employed, a ballast which serves to regulate the electric current through the lamp or lamps, and in some instances a transparent or translucent diffuser. A typical fixture would be equipped with two fluorescent lamps, each four feet long, although other types of lamps are available.
An emergency lighting unit for use with such a lighting fixture consists of a rechargeable electric storage battery, a battery charging means, a transfer circuit including means for sensing the onset of a power failure, and a dc to ac inverter to provide alternating current at a voltage suitable for maintaining the electric discharge lamp lit during the power failure. In operation, the battery is normally kept in the fully charged condition by the battery charging circuit, using power derived from the electric supply line. When a power failure occurs, the transfer circuit senses this condition and is activated so as to energize the inverter, the dc input circuit of which is then connected to the battery, and the ac output circuit is connected to the lamp to be maintained.
Such emergency lighting units require the connection of at least four wires between the unit and the electrical circuits in the lighting fixture. Two of these wires are required for connection to the ac electric line power supply (for the purpose of feeding the battery charger) and at least two additional wires are required to connect the output circuit of the inverter to the lamp to be maintained.
Some emergency lighting units are designed for installation inside the lighting fixture. With such units, it is necessary to partially dismantle the lighting fixture in order to mount the emergency lighting unit inside it, and to make the various electrical wiring connections. A typical such unit designed for mounting inside a lighting fixture is described in my Canadian Pat. No. 887,282, issued Nov. 30, 1971.
Other types of emergency lighting units are designed for mounting outside the housing of the lighting fixture. One particular example of this type consists of a tubular housing, containing the battery, battery charger, transfer circuit and inverter, equipped with a lamp socket at one end and with two contact pins suitable for insertion in a lamp socket at the other end. The length of the tubular housing is twelve inches, and it can be mounted, in tandem with a three-foot long fluorescent lamp in the space previously occupied by a four-foot lamp in a fluorescent lighting fixture. Wire leads emanating from the emergency lighting unit must be connected, through a hole or holes drilled in the housing of the lighting fixture, to make the required electrical connections to operate the unit.
Both the in-fixture and the external emergency lighting units described above suffer from the disadvantage of requiring wiring connections between the unit and the components inside the lighting fixture. The necessity for these wiring connections means that the services of a qualified electrician for a considerable period of time are required, which results in considerable cost for installing the emergency lighting unit. Furthermore, if it is desired to move the emergency lighting unit from one lighting fixture to another then additional time and expense are involved.